1. Field of the Invention
The present invention relates to a terminal apparatus, a terminal apparatus controlling method, and a control program. More particularly, the invention relates to a terminal apparatus connected to a host device illustratively through a USB port.
2. Description of the Related Art
In recent years, many terminal apparatuses including digital still cameras and digital audio players have adopted the USB (Universal Serial Bus) as the interface for connecting with other devices.
With recording media getting ever larger in capacity, the above-mentioned type of terminal apparatus connected with a host device through a USB port has an increasing number of files to be exchanged with the latter. The result is a tendency toward prolonged connection time for the devices involved.
The above type of terminal apparatus contains a battery. While being connected with the host device, the terminal apparatus keeps consuming power from its battery. As the connection time is prolonged, as has been the case with the terminal apparatuses as mentioned above, the battery can go dead during the connection.
It might happen that the terminal apparatus compatible with a USB mass storage class recognized as a drive by the host device has the battery run out during file exchange with the host. If that happens in reality, there is a possibility that the data written on the recording medium of the terminal apparatus is destroyed.
For that reason, the user must keep a constant watch on the remaining battery level of the terminal apparatus while operating the host device. This has been a disadvantage to the user.
Meanwhile, according to the USB standard, it is possible for the host device to power its terminal apparatus via a signal line called VBUS. Utilizing this facility can eliminate the above-mentioned disadvantage to the user.
More specifically, the USB standard defines two kinds of devices utilizing VBUS power: devices that operate only on VBUS power, and devices operating on both VBUS power and self-supplied power.
The devices operating only on VBUS power are called bus-powered devices (Bp devices). The devices operating on both VBUS power and self-supplied power are called self-powered devices (Sp devices).
Immediately after being connected with the host device, the Bp device may draw currents of up to only 100 mA. When authenticated by the host device, the Bp device is allowed to draw currents of up to 500 mA.
On the other hand, the Sp device is allowed to draw currents of only up to 100 mA. That is, the terminal apparatus operating from its battery is not allowed to draw currents exceeding 100 mA from the host device.
Meanwhile, many of the terminal apparatuses subject to power constraints cannot operate at the current of 100 mA. If allowed to draw currents of up to 500 mA, more and more terminal apparatus may conceivably operate only on the power (5 V, 500 mA) from the host device.
However, even if the terminal apparatus notifies the host device that it is a Bp device, in order to draw the 500 mA current from the latter, the terminal apparatus must initially operate at the 100 mA current until it is authenticated by the host device.
As a solution to the above bottleneck, there has been proposed a terminal apparatus supplementing its main control unit operating at the 500 mA current with a separate control unit which communicates with the host device and which operates at the 100 mA current (e.g., see Japanese Patent Laid-open No. 2004-69997).